Conventionally, there has been known femtosecond laser light, which has the pulse width of a 10−13 second order as a laser light source of a sub-picosecond or less.
When femtosecond laser light being laser light that is emitted from such femtosecond laser, which is a femtosecond laser having the pulse width of about 100 to 150 femtoseconds (“laser light that is emitted from femtosecond laser” shall be referred to as “femtosecond laser light” in this specification), for example, is focused on the inside of a transparent material with respect to the femtosecond laser light (a transparent material with respect to predetermined femtosecond laser light shall be appropriately referred to simply as a “transparent material” in this specification) via a lens, there is known a phenomenon that multiphoton absorption occurs only at a focal spot being a light focus position and processing such as the modification or the working of the focal spot inside the transparent material can be performed.
In recent years, report has been made that refraction index is changed, crystal precipitation is performed, or a micro-void is generated inside a glass material that is transparent with respect to the femtosecond laser light, with the use of such phenomenon. Furthermore, report has been made regarding the fabrication of an optical waveguide, a three-dimensional memory, a photonic crystal, or a micro-channel.
Meanwhile, regarding the structure of the above-described micro-void or the like that has been fabricated using the above-described phenomenon, when observation was made for the cross-sectional shape in a direction parallel to the advance direction of the beam of femtosecond laser light, it was made clear that the cross-sectional shape greatly depended on the longitudinal direction of the femtosecond laser light at the focal spot of the femtosecond laser light, that is, the spatial intensity distribution in a direction parallel to the advance direction of the beam of femtosecond laser light.
Specifically, according to the calculation result of the inventors of this application, when the femtosecond laser light is focused using an objective lens having the numerical aperture (NA) of 0.46 (“numerical aperture=0.46” is equivalent to magnification of ×20), the spatial intensity distribution in the direction parallel to the advance direction of the beam of femtosecond laser light at the focal spot (spatial intensity distribution on a Y-Z plane in FIG. 2) should be the intensity distribution of a lengthwise elliptical shape extending in the advance direction of the beam of femtosecond laser light, as shown in FIG. 1(a).
As a result, the processed shape such as a modified shape and a worked shape inside the transparent material became the lengthwise elliptical shape extending in the advance direction of the beam of femtosecond laser light by suffering from the spatial intensity distribution in the direction parallel to the advance direction of the beam of femtosecond laser light at the focal spot inside the transparent material.
However, there existed a problem that it was not desirable that the processed shape such as a modified shape and a worked shape inside the transparent material became the lengthwise elliptical shape extending in the advance direction of the beam of femtosecond laser light when industrial fabrication and usage of micro-channel or optical waveguide are taken in consideration. Specifically, as the processed shape such as a modified shape and a worked shape inside the transparent material, a shape whose cross-sectional shape in the direction parallel to the advance direction of the beam of femtosecond laser light is a substantially perfect circle was required.
Herein, in order to obtain a shape whose cross-sectional shape in the direction parallel to the advance direction of the beam of femtosecond laser light is a substantially perfect circle as the processed shape such as a modified shape and a worked shape inside the transparent material, the spatial intensity distribution in the direction parallel to the advance direction of the beam of femtosecond laser light should be a shape of a substantially perfect circle.
Generally, as a method of obtaining a shape whose cross-sectional shape in the direction parallel to the advance direction of the beam of femtosecond laser light is a substantially perfect circle as the processed shape such as a modified shape and a worked shape inside the transparent material while obtaining the spatial intensity distribution of a substantially perfect circle in the direction parallel to the advance direction of the beam of femtosecond laser light, the use of an objective lens having a large numerical aperture is considered.
Certainly, when the femtosecond laser light is focused on the inside of the transparent material using an objective lens having the numerical aperture of 1 or more and the processing such as the modification and working inside the transparent material is performed, there is known that a shape whose cross-sectional shape in the direction parallel to the advance direction of the beam of femtosecond laser light is the shape of substantially perfect circle is obtained as the processed shape such as a modified shape and a worked shape on the focal spot (Hiroaki MISAWA et al., Proc. SPIE Vol. 4088, p 29-32).
However, when the femtosecond laser light is focused on the inside of the transparent material using the objective lens having the numerical aperture of 1 or more, a working distance (the working distance is the distance between the objective lens and a sample that is the transparent material to be modified or worked) becomes several hundred μm or less (the working distance is 200 to 300 μm when the femtosecond laser light is focused on the inside of the transparent material using an objective lens having the numerical aperture of 1), so that a problem was pointed out that the processing such as the modification and the working with respect to a region at a deep position inside the transparent material was impossible.
The present invention has been created in view of the above-described problems that prior art has, and it is an object of the present invention to provide a method and a device for processing the inside of a transparent material, which is capable of obtaining a cross-sectional shape in a direction parallel to the advance direction of the beam of femtosecond laser light is a substantially perfect circle as the processed shape such as a modified shape or a worked shape on a light focus position being a light focus position, even if the femtosecond laser light is focused on the inside of the transparent material using a condenser lens of a relatively small numerical aperture, by which the working distance of several mm or more can be secured.